Author Harper Lee never specifically states that Boo saved the children from Bob's attack. But reading between the lines and using statements made by Scout and Sheriff Tate, it becomes obvious that Boo is the man who comes to their rescue and saves Jem and Scout. We do know that Scout saw a man "carrying Jem"--he was "pantin' and--coughing fit to die"--who then joined Atticus and "took Jem inside." Scout does not recognize the man, believing him to be "some countryman I did not know." Later, Scout points out the man to Sheriff Tate, and it is only then that she realizes it must be Boo.
Although Tate maintains (falsely) that "Bob Ewell fell on his knife," he makes it clear that Boo must have been the man who both rescued the children and killed Bob.
"I never heard tell that it's against the law for a citizen to do his utmost to prevent a crime from being committed, which is exactly what he did... (Chapter 30)
Tate adds that Boo is "the one man who's done you and this town a great service," and Scout tells Atticus that "I understand... Mr. Tate was right." Atticus thanks Boo "for my children," and later still, Scout acknowledges that Boo has finally come out of the Radley house when "Boo's children needed him" most.